The present invention relates to a system using a power converter with switching operation, for example, a system for driving a load such as a motor by output of the power converter.
Today, the power converter controlled with the switching operation is used in various devices such as a motor drive unit and required to have a robust and precise performance. To response the demand, switching characteristics of power elements forming the power converter has been improved and a considerably high switching speed of 10 to 100 nanoseconds is achieved. As a result, frequency of EMI (electromagnetic interference) noise generated from the system using the power converter becomes higher and thus the noise is enterable readily into other electronic or information devices via floating capacitance as a common mode noise, causing electromagnetic disturbance.
The common mode current (leak current) flowing within the motor drive system flows via the floating capacitance distributed variously in the system. To restrain such common mode current, it is essential to grasp the behavior of the common mode noise (voltage and current) in the drive system and establish a circuit model reflecting the behavior.
FIG. 8 is a view showing a schematic configuration of the motor drive system as an example of the system using the power converter with the switching operation. The motor drive system of FIG. 8 includes an AC power supply 1, AC reactor 2, power converter 3, and motor 4. The AC reactor 2, frame of the motor 4 (not shown), and the ground line of the power converter 3 (not shown) are connected to the ground 5. Herein, the frame means a structure supporting, the entire device covered by a conductor in electrically insulated condition from the electrified area of the device.
The power converter 3 receives the power supplied from the AC power supply 1 via the AC reactor 2 and converts it into a power of an optional frequency and voltage. The power converter 3 has a converter (rectifier) 31 for converting the input AC power to DC voltage, smoothing capacitor 32 for smoothing the output DC voltage of the converter 31, and inverter 33 for converting the smoothed DC voltage to AC voltage as basic components. The components are mounted on a circuit board 30 on which the converter 31 and inverter 33 are connected each other via DC main circuit conductors 300n, 300p formed on the circuit board 30. A cooling fin 34 is installed on an element housing framing the converter 31 and inverter 33. The cooling fin 34, restraining the temperature rise of the elements, is electrically connected to the ground line (not shown) of the circuit board 30.
There are two types of the EMI noise in such system. One is a normal mode noise generated by differential voltage from the DC main circuit conductors 300n, 300p between the converter 31 and inverter 33, the other is the common mode noise generated by the common mode current flowing through parasitic capacitance distributed in the motor drive system.
There are three types of the common mode current as shown in FIG. 8: current Ic2 leaking from the element housing of the power converter 3 to the ground via the cooling fin 34, current Ic3 leaking to the ground via the frame of the AC reactor 2, and current Ic1 leaking via the motor frame. Since the common mode current flows via the floating capacitance as shown in FIG. 8, high-frequency noise component flows as the leak current. In FIG. 8, symbols for the floating capacitance are omitted.
The normal mode noise can be restrained by inserting a noise filter in a DC line between the converter and inverter. For reducing the common mode noise, a method, employing a common mode transformer or active common mode chancellor between the inverter 33 and motor 4, has been proposed.
However, as it is expected that the frequency of the EMI noise, to which measures must be taken, will further increase, there is a possibility in the related art methods that a new current path inducing the common mode noise is formed through frames of an additional apparatus for the measures. Also, since those are the methods that restrain the noise by switching of the power elements in a condition that the apparatus for the measures is placed on a power transfer line between the inverter 33 and motor 4, there is a possibility of the new EMI noise generated by this switching operation. Also, the additional circuit for reducing the normal mode noise should be preferably avoided if possible.